Haloalkanes and Haloarenes · Lesson 2 of 16
Naming and Structural Isomerism
“Build reliable systematic names and enumerate distinct structures using skeletons and symmetry.”
• Choose and number a parent structure while accounting for unsaturation and all substituents. • Translate common organohalogen names into systematic names and structures. • Enumerate haloalkane constitutional isomers without double-counting equivalent positions. • Explain why constitutional and stereochemical differences must be counted separately.
Naming is a way to communicate a structure
In systematic names, fluoro-, chloro-, bromo- and iodo- are substituent prefixes. Identify the parent chain or ring, locate multiple bonds and other substituents, then choose numbering that follows the applicable priority rules. A low halogen number cannot override the low locant required for a parent double or triple bond. When equivalent numberings remain, compare complete locant sets at the first point of difference; alphabetic order resolves appropriate remaining substituent ties, not every numbering problem.
| Condensed structure | Common name | Systematic name |
|---|---|---|
| CH₃CH₂CHClCH₃ | sec-Butyl chloride | 2-Chlorobutane |
| (CH₃)₃CCH₂Br | Neopentyl bromide | 1-Bromo-2,2-dimethylpropane |
| (CH₃)₃CBr | tert-Butyl bromide | 2-Bromo-2-methylpropane |
| CH₂=CHCl | Vinyl chloride | Chloroethene |
| CH₂=CHCH₂Br | Allyl bromide | 3-Bromoprop-1-ene |
| o-ClC₆H₄CH₃ | o-Chlorotoluene | 1-Chloro-2-methylbenzene |
| C₆H₅CH₂Cl | Benzyl chloride | (Chloromethyl)benzene |
| CH₂Cl₂ | Methylene chloride | Dichloromethane |
| CHCl₃ | Chloroform | Trichloromethane |
| CHBr₃ | Bromoform | Tribromomethane |
| CCl₄ | Carbon tetrachloride | Tetrachloromethane |
| CH₃CH₂CH₂F | n-Propyl fluoride | 1-Fluoropropane |
For disubstituted benzene, ortho means adjacent 1,2 positions, meta means 1,3, and para means opposite 1,4. The common name p-bromochlorobenzene corresponds to 1-bromo-4-chlorobenzene. With three identical substituents at alternating positions, the explicit name is 1,3,5-tribromobenzene. A ring drawing or locanted name is more precise than simply writing C₆H₃Br₃.
Choose the parent before assigning low numbers
Problem
Name CH₃CH=CHCH(Br)CH₃ and CH₂=C(CH₃)CH(Br)CH₃.
- 1.For the first molecule choose a five-carbon chain containing C=C. Number to give the double bond locant 2 rather than 3. Br is then at C4: 4-bromopent-2-ene.
- 2.For the second choose the four-carbon chain containing the double bond. Number from CH₂ so the double bond begins at C1. Methyl is at C2 and bromo at C3: 3-bromo-2-methylbut-1-ene.
- 3.Do not reverse the chain merely to make bromo have a smaller number; the multiple-bond locant is selected first in these examples.
| Further structure | Name | What to check |
|---|---|---|
| CH₃CH=C(CH₃)CH(Br)CH₃ | 4-Bromo-3-methylpent-2-ene | Double bond before halogen locant |
| BrCH₂C(CH₃)=CHCH₃ | 1-Bromo-2-methylbut-2-ene | Same double-bond locant; substituent comparison |
| BrCH₂CH=CHCH₃ | 1-Bromobut-2-ene | Allylic Br |
| CH₂=C(CH₃)CH₂Br | 3-Bromo-2-methylprop-1-ene | Select chain containing C=C |
| BrCH₂C≡CCH₂Cl | 1-Bromo-4-chlorobut-2-yne | Equal multiple-bond locant; alphabetic tie |
| CHF₂C(Br)(Cl)F | 1-Bromo-1-chloro-1,2,2-trifluoroethane | Compare full substituent locant sets |
Problem
Compare the possible numberings of CH₃CH(Cl)CH(Br)CH₃.
- 1.Both ends give substituent positions 2 and 3, so a complete locant-set comparison remains tied.
- 2.Bromo is cited before chloro alphabetically and receives the lower number in this tie: 2-bromo-3-chlorobutane.
- 3.Prefixes di-, tri- and tetra- do not normally determine alphabetical order. Alphabetic priority is used only after the relevant structural numbering criteria have been applied.
Longer displayed formulas may conceal a longer parent than the first row of letters suggests. CH₃CH₂CH(CH₃)CH(C₂H₅)Cl contains a six-carbon path passing through the ethyl branch; its name is 3-chloro-4-methylhexane. Likewise CH₃CH₂C(CH₃)₂CH₂I is 1-iodo-2,2-dimethylbutane. The carbon that bears iodine is primary even though the next carbon has four carbon neighbours. For ring-containing structures use parentheses for a compound substituent, such as 2-(2-chlorophenyl)-1-iodooctane, so the ring locant cannot be confused with the chain locant.
Enumerate structures systematically, using symmetry
For C₅H₁₁Br, start with the three C₅ carbon skeletons rather than moving Br randomly through drawings. Pentane has three distinct replacement positions because its two ends are equivalent. 2-Methylbutane has four distinguishable substitution sites. 2,2-Dimethylpropane has one because all four methyl groups are equivalent. The total is 3+4+1=8 constitutional isomers. Turning a drawing around does not create a ninth structure.
| C₅H₁₁Br structure | Systematic name | Degree |
|---|---|---|
| BrCH₂CH₂CH₂CH₂CH₃ | 1-Bromopentane | 1° |
| CH₃CHBrCH₂CH₂CH₃ | 2-Bromopentane | 2° |
| CH₃CH₂CHBrCH₂CH₃ | 3-Bromopentane | 2° |
| BrCH₂CH₂CH(CH₃)CH₃ | 1-Bromo-3-methylbutane | 1° |
| CH₃CHBrCH(CH₃)CH₃ | 2-Bromo-3-methylbutane | 2° |
| CH₃C(Br)(CH₃)CH₂CH₃ | 2-Bromo-2-methylbutane | 3° |
| BrCH₂CH(CH₃)CH₂CH₃ | 1-Bromo-2-methylbutane | 1° |
| BrCH₂C(CH₃)₃ | 1-Bromo-2,2-dimethylpropane | 1° |
Some of these structures have stereocentres and can exist as more than one stereoisomer. Eight is the number of constitutional connectivities, not the total number of all stereoisomeric molecules. The distinction matters when a question asks for structural isomers only. We will treat three-dimensional configurations separately rather than silently counting mirror images as new connectivities.
Problem
Enumerate the structural isomers of C₄H₉Br.
- 1.The straight butane skeleton has two inequivalent sites: 1-bromobutane, CH₃CH₂CH₂CH₂Br, and 2-bromobutane, CH₃CHBrCH₂CH₃.
- 2.The branched 2-methylpropane skeleton gives 1-bromo-2-methylpropane, (CH₃)₂CHCH₂Br, and 2-bromo-2-methylpropane, (CH₃)₃CBr.
- 3.These are two primary, one secondary and one tertiary bromide. The enantiomers of 2-bromobutane are not additional constitutional isomers.
Problem
List dichloro derivatives of the propane skeleton.
- 1.The end carbons are equivalent, so 1,1 and 3,3 substitution describe the same connectivity, as do 1,2 and 2,3.
- 2.The four distinct structures are CHCl₂CH₂CH₃, ClCH₂CHClCH₃, ClCH₂CH₂CH₂Cl and CH₃CCl₂CH₃.
- 3.Their names are 1,1-, 1,2-, 1,3- and 2,2-dichloropropane. Only the 1,2 case is vicinal; the 1,1 and 2,2 cases are geminal.
Reading a name back into a structure
For 2-chloro-3-methylpentane, draw five carbons, put Cl on C2 and CH₃ on C3: CH₃CHClCH(CH₃)CH₂CH₃. For 1,4-dibromobut-2-ene draw BrCH₂CH=CHCH₂Br. For 4-tert-butyl-3-iodoheptane, retain the seven-carbon parent and attach I at C3 and C(CH₃)₃ at C4. In a name such as 1-bromo-4-(butan-2-yl)-2-methylbenzene, place Br and both carbon substituents on the specified ring positions; the side chain attaches through its second carbon. A bond-line drawing must preserve that attachment, not merely the total carbon count.
Problem
Read branched and ring-containing formulas without losing the longest chain or a ring position.
- 1.CH₃C(C₂H₅)₂CH₂Br contains a five-carbon path between the two ethyl ends; one suitable name is 3-(bromomethyl)-3-methylpentane. A short drawn horizontal path is not automatically the parent chain.
- 2.For m-ClCH₂C₆H₄CH₂C(CH₃)₃, the two ring substituents are meta: 1-(chloromethyl)-3-(2,2-dimethylpropyl)benzene describes the connectivity. The Cl is benzylic, not directly aryl.
- 3.For CH₃C(p-ClC₆H₄)₂CH(Br)CH₃, a chain-based name is 3-bromo-2,2-bis(4-chlorophenyl)butane: compare the full locant sets 2,2,3 and 2,3,3 before alphabetic ordering.
- 4.For (CH₃)₃CCH=C(Cl)C₆H₄I-p, a chain-based description is 1-chloro-1-(4-iodophenyl)-3,3-dimethylbut-1-ene. Parentheses keep ring positions separate from chain positions; no unspecified E/Z assignment is added.
| Condensed connectivity | One suitable systematic description | C–X class |
|---|---|---|
| (CH₃)₂CHCH(Cl)CH₃ | 2-Chloro-3-methylbutane | Secondary alkyl |
| CH₃CH₂CH(CH₃)CH(C₂H₅)Cl | 3-Chloro-4-methylhexane | Secondary alkyl |
| CH₃CH₂C(CH₃)₂CH₂I | 1-Iodo-2,2-dimethylbutane | Primary alkyl |
| (CH₃)₃CCH₂CH(Br)Ph | 1-Bromo-3,3-dimethyl-1-phenylbutane | Secondary benzylic |
| CH₃CH(CH₃)CH(Br)CH₃ | 2-Bromo-3-methylbutane | Secondary alkyl |
| CH₃C(C₂H₅)₂CH₂Br | 3-(Bromomethyl)-3-methylpentane | Primary alkyl |
| CH₃C(Cl)(C₂H₅)CH₂CH₃ | 3-Chloro-3-methylpentane | Tertiary alkyl |
| CH₃CH=C(Cl)CH₂CH(CH₃)₂ | 3-Chloro-5-methylhex-2-ene | Vinylic |
| CH₃CH=CHC(Br)(CH₃)₂ | 4-Bromo-4-methylpent-2-ene | Tertiary allylic |
| p-ClC₆H₄CH₂CH(CH₃)₂ | 1-Chloro-4-(2-methylpropyl)benzene | Aryl |
| m-ClCH₂C₆H₄CH₂C(CH₃)₃ | 1-(Chloromethyl)-3-(2,2-dimethylpropyl)benzene | Primary benzylic |
| o-BrC₆H₄CH(CH₃)CH₂CH₃ | 1-Bromo-2-(butan-2-yl)benzene | Aryl |
| Specified name | Structure or explicit ring attachment |
|---|---|
| 2-Chloro-3-methylpentane | CH₃CH(Cl)CH(CH₃)CH₂CH₃ |
| p-Bromochlorobenzene | 1-Br-4-ClC₆H₄; Br and Cl opposite on the ring |
| 1-Chloro-4-ethylcyclohexane | Saturated six-membered ring: Cl at C1, CH₂CH₃ at C4 |
| 2-(2-Chlorophenyl)-1-iodooctane | ICH₂CH(o-ClC₆H₄)(CH₂)₅CH₃ |
| 2-Bromobutane | CH₃CH(Br)CH₂CH₃ |
| 4-tert-Butyl-3-iodoheptane | CH₃CH₂CH(I)CH[C(CH₃)₃]CH₂CH₂CH₃ |
| 1-Bromo-4-(butan-2-yl)-2-methylbenzene | Ring C1–Br, C2–CH₃, C4–CH(CH₃)CH₂CH₃ |
| 1,4-Dibromobut-2-ene | BrCH₂CH=CHCH₂Br; E/Z unspecified |
Quick Quiz
Quiz
How many constitutional C₅H₁₁Br isomers are there?
Explanation: The three C₅ skeletons supply 3,4 and 1 inequivalent bromination sites.
What is CH₂=CHCH₂Br called systematically?
Explanation: The alkene receives the lower locant before the bromo position is assigned.
Which name correctly describes CH₃CH(Cl)CH(Br)CH₃?
Explanation: The 2,3 locant set ties, so the alphabetically first bromo prefix gets 2.
Which pair represents the same dichloropropane connectivity?
Explanation: Reversing propane exchanges its equivalent end carbons.
Do enantiomers count as additional constitutional isomers?
Explanation: Constitutional isomerism concerns which atoms are bonded, not their spatial arrangement.
Practice Problems
- Name and classify CH₃CH(CH₃)CHClCH₃ and CH₃C(Cl)(C₂H₅)CH₂CH₃.
- Name BrCH₂C≡CCH₂Cl and explain numbering.
- Draw the structure implied by 1-chloro-4-ethylcyclohexane.
- Explain why (CCl₃)₃CCl has ten chlorines and give a systematic structural description.
- Explain the three-plus-four-plus-one counting for C₅H₁₁Br.
- Write 1,4-dibromobut-2-ene and explain why E/Z cannot be inferred from the name alone.
The first is 2-chloro-3-methylbutane, a secondary alkyl chloride. The second has a five-carbon parent running between the two ethyl ends and is 3-chloro-3-methylpentane, a tertiary chloride. Locate the longest chain and then count carbon neighbours at C–Cl.
Key Takeaways
• Systematic names encode parent structure, multiple bonds and substituent positions. • Numbering follows structural priority rules before alphabetic tie-breaking. • Common names should be translated into explicit connectivity. • Equivalent positions must be recognised before counting isomers. • C₅H₁₁Br has eight constitutional isomers and C₄H₉Br has four. • Constitutional isomers and stereoisomers are different counting problems. • A name without stereodescriptors does not specify an unmentioned configuration.